A wide variety of memory devices can be used to maintain and store data and instructions for various computers and similar systems. In particular, flash memory is a type of electronic memory media that can be rewritten and that can retain content without consumption of power. Unlike dynamic random access memory (DRAM) devices and static random memory (SRAM) devices in which a single byte can be erased, flash memory devices are typically erased in fixed multi-bit blocks or sectors. Flash memory technology can include NOR flash and NAND flash, for example. NOR flash evolved from electrically erasable read only memory (EEPROM) chip technology, in which, unlike flash, a single byte can be erased; and NAND flash evolved from DRAM technology. Flash memory devices are less expensive and denser as compared to many other memory devices, meaning that flash memory devices can store more data per unit area.
NOR Flash memory architecture is an array of Flash EEPROM cells (floating gate devices) which are divided into a plurality of sectors. Further, the memory cells within each sector are arranged in rows of wordlines and columns of bitlines intersecting the rows of wordlines. The source region of each cell transistor within each sector is tied to a common node. Therefore, all of the cells within a particular sector can be erased simultaneously and erasure may be performed on a sector-by-sector basis. The control gates of the cell transistors are coupled to wordlines, and the drains thereof are coupled to bit lines.
Flash memory devices can include a number of sectors that can include word lines and bit lines associated with memory cells to or from which data can be written and/or read. Each sector can include a plurality of memory cells. Further, each sector can include a boost-strap node to which a voltage can be applied in order to facilitate writing or reading data to/from the flash memory. During a read operation, a voltage source can supply a voltage, so that the respective voltage levels at the boost-strap nodes respectively associated with each of the sectors can be increased to a desired voltage level. The boost-strap node can facilitate enabling the signal at the vertical word line to pass to the memory cell word line to facilitate the reading of data from the memory cell associated with the memory cell word line. The desirable amount of time to raise the boost-strap node voltage to the desired voltage level can be limited. Further, parasitic elements can delay the increase of the voltage level in the boost-strap nodes and/or introduce inconsistencies between the voltage ramp rates of the boost-strap nodes of respective sectors, where such parasitic elements can include resistance in the channel associated with the boost-strap nodes as well as stray capacitance, which can result from the routing of the circuitry in the device.
As is generally known in the field of semiconductor memory devices and other semiconductor integrated circuits, it is often required to generate internally voltages that are greater than an external voltages, also known as off-chip power supply voltages. For example, it is known in flash EEPROMs that a first high voltage of about +5V is needed to be produced for the read mode of operation of memory cells. Also, a second high voltage of about +10V is needed to be produced for the program mode of operation of the flash memory cells. To meet this requirement, the semiconductor memories also generally include one or more internal voltage boosting circuits for generating output signals boosted to be higher than an external supply voltage.
Conventional boosted voltage circuits generate a boosted voltage and apply the boosted voltage to a word line for read mode operations of memory cells. In such conventional boosted voltage circuit, the boosted voltage varies with supply voltage levels, process corners, and temperature. Thus, the boosted voltage is not accurate and causes errors to occur during read mode operations of memory cells. Such variations of the boosted voltage degrade an ability in a read mode circuitry to discriminate accurately whether or not a cell is programmed. In addition, as device densities and memory speed requirements continue to increase, a speed requirement of the boosted voltage circuit may need to increase to keep pace with a remainder of the memory circuit. Further, as supply voltage levels decrease with the higher density architectures, conventional boosted voltage circuits may be inadequate to supply a required boost voltage. As supply voltage levels decreases with the higher density architectures, a single stage voltage booster circuit may be inadequate to supply the required boosted voltage.